I amCustomerand I am happy to help answer your questions on your Sea Doo GTI RFI.
As far as winterizing it and anti freeze installation, your particular ski does not need that. The cooling system on your ski uses what is called raw water, or water picked up from the water it is being operated in, to cool the engine. It is pumped through the engine, and expelled with the exhaust.
The Anti Freeze would be associated with engines that have a closed cooling system. They use a heat exchanger (similar to a radiator on a car) that uses coolant circulated through the engine, then the raw water flows through the heat exchanger to cool it. The reason they typically use a closed system heat exchanger is to keep salt water out of the engine, due to the corrosiveness of the salt water.
I am curious where you heard something about anti freeze as a part of the winterizing process. Personal Water Craft, or PWC for short, typically do not use closed systems for engine cooling. They almost exclusively use the raw water for cooling with no coolant storage designed into their systems.
One thing you do want to do is make sure you have evacuated as much water from the exhaust system as possible, You can do that by revving the engine a couple of times after you shut the flush hose off, especially with the nose of the craft elevated.
If you have any more questions, feel free to ask.